The invention relates to a process for the preparation of xcex5-caprolactam starting from 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers or polymers of these compounds or mixtures comprising at least two of these compounds.
Such a process is described in JP-A-51039684. This patent publication describes a process to prepare xcex5-caprolactam from nylon-6 polymer using an adduct of xcex5-caprolactam and terephthalic acid, isophthalic acid or adipic acid as depolymerisation catalyst.
A disadvantage is that the catalyst is a process-foreign compound resulting in xcex5-caprolactam containing undesired impurities. Such impurities would not be present when the process is performed in the substantial absence of such process-foreign compounds.
The object of the present invention is a process for the preparation of xcex5-caprolactam in which the above mentioned disadvantage is avoided or at least reduced.
This object is achieved in that the process is performed in the presence of N-(5-carboxypentyl)-xcex5-caprolactam and/or derivative thereof in an amount of less than 50 wt. % and more than 0.1 wt. % (based on the total reaction mixture).
It has been found that the rate of the reaction of the process according to the invention is significantly higher than the rate of reaction of a process for the preparation of xcex5-caprolactam starting from the above mentioned compounds performed in the substantial absence of N-(5-carboxypentyl)-xcex5-caprolactam and/or derivative thereof or performed in the presence of higher amounts of N-(5-carboxypentyl)-xcex5-caprolactam and/or derivative thereof. This is advantageous because smaller reactor equipment can be used and/or less residence time is needed resulting in less degradation reactions. Degradation reactions result in yield loss and/or in the formation of undesirable by-products making the purification of xcex5-caprolactam more difficult. Another advantage is that N-(5-carboxypentyl)-xcex5-caprolactam and/or derivative thereof is a process-own compound so that the process of the invention does not result in xcex5-caprolactam containing extra impurities due to the use of a process-foreign catalyst compound.
JP-A-45022946 describes a process for the preparation of xcex5-caprolactam by reacting N-(5-carbamoylpentyl)-xcex5-caprolactam with aqueous ammonia at a temperature of 350xc2x0 C. The compound N-(5-carboxypentyl)-xcex5-caprolactam itself is known from JP-A-45022946. The use of N-(5-carboxypentyl)-xcex5-caprolactam or derivative thereof as reaction rate enhancing compound in a process for preparing xcex5-caprolactam from 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers or polymers of these compounds or mixtures comprising at least two of these compounds is however not disclosed. N-(5-carboxypentyl)-xcex5-caprolactam is said to be obtained by processing the distillation residue that is obtained in the distillation of crude xcex5-caprolactam obtained by the reaction of caprolactone and aqeous ammonia. The N-(5-carboxypentyl)-xcex5-caprolactam must subsequently be converted into N-(5-carbamoylpentyl)-xcex5-caprolactam before the latter compound is reacted with aqueous ammonia at a temperature of 350xc2x0 C. to obtain xcex5-caprolactam.
With N-(5-carboxypentyl)-xcex5-caprolactam is meant a compound with the following structural formula: 
With N-(5-carbamoylpentyl)-xcex5-caprolactam is meant a compound with the following structural formula: 
In the present application, the term N-(5-carboxypentyl)-xcex5-caprolactam derivative include compounds with the following structural formula: 
or with the following structural formula: 
wherein Rxe2x80x2 is NH2 or OR1; n is at least 1; Rxe2x80x3 is OH, NH2 or OR1; R1 is preferably an organic group with 1 to 20 carbon atoms and more preferably with 1 to 6 carbon atoms. The organic group is an alkyl, cycloalkyl, aryl or aralkyl group. More preferably R1 is an alkyl group wit 1 to 6 carbon atoms. Examples of R1 groups include methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, isobutyl, cyclohexyl, benzyl and phenyl. By preference R1 is methyl or ethyl.
The N-(5-carboxypentyl)-xcex5-caprolactam and N-(5-carbamoylpentyl)-xcex5-caprolactam can for example be prepared by the method as described in for example JP-A-45022946. The N-(5-carboxypentyl)-xcex5-caprolactam may be formed or can for example be prepared from di-(5-carboxypentyl)amine using the conditions as applied in the process of the invention according to the following reaction scheme: 
The di-(5-carboxypentyl)amine may be formed or can for example be prepared from 6-aminocaproic acid using the conditions as applied in the process of the invention according to the following reaction scheme: 
The di-(5-carboxypentyl)amine may be formed or can for example be prepared from 6-aminocaproic acid and a 5-formylvaleric acid ester using the conditions as described in for example WO-A-9835938 according to the following reaction scheme: 
The compound with formula 
in which Rxe2x80x2 is as defined above, may be formed or can for example be prepared from 6-aminocaproic acid and 6-aminocaproamide (in case Rxe2x80x2 is xe2x80x94NH2) or from 6-aminocaproic acid and 6-aminocaproate ester (in case Rxe2x80x2 is xe2x80x94OR1) using the conditions as applied in the process of the invention.
In case the term N-(5-carboxypentyl)-xcex5-caprolactam is stated below, it should be read as N-(5-carboxypentyl)-xcex5-caprolactam and/or derivative thereof.
The N-(5-carboxypentyl)-xcex5-caprolactam may be added at the beginning of the process according to the present invention. The N-(5-carboxypentyl)-xcex5-caprolactam may also already be present in a reaction mixture obtained in a previous process step, which also contains at least one of 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers or polymers of these compounds.
Is has been found that the presence of N-(5-carboxypentyl)-xcex5-caprolactam in the reaction mixture in an amount of between 0.1 and 50 wt. % (based on the total reaction mixture) results in an increase of the reaction rate. With xe2x80x9cthe total reaction mixturexe2x80x9d is meant the starting compound or mixture of starting compound (optionally containing water) which is fed to the process according to the invention. It has been found that the presence of N-(5-carboxypentyl)-xcex5-caprolactam in the reaction mixture in an amount higher than 50 wt. % results in a decrease of the reaction rate. The amount of N-(5-carboxypentyl)-xcex5-caprolactam in the reaction mixture is preferably between 0.1 wt. % and 10 wt. % (based on the total reaction mixture).
The amount of N-(5-carboxypentyl)-xcex5-caprolactam can also be kept at and/or brought to the desired value within the above general and preferred ranges by adding or by removing N-(5-carboxypentyl)-xcex5-caprolactam, for example by purging.
The process according to the invention may be performed in the gas phase by contacting the starting compound(s) (6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers or polymers of these compounds or mixtures comprising at least two of these compounds) with superheated steam at a temperature of between 150 and 500xc2x0 C. and a pressure of between 0.1 and 2 MPa.
An example of a possible gaseous phase process is described in WO-A-9837063. The process may also be performed in the liquid phase at elevated temperatures and super atmospheric pressures such as for example described in U.S. Pat. No. 4,730,040, EP-A-729944 and WO-A-9809944.
The process according to the invention is preferably performed in the gas phase. The gas phase processes are advantageous because xcex5-caprolactam is obtained in a gaseous steam phase in which no or partically no oligomers are present. This is advantageous because the purification of xcex5-caprolactam is easier. The gas phase is preferably performed as described in WO-A-9837063, the complete disclosure is incorporated herein as reference. The gas phase process is preferably performed at a temperature of between 250 and 400xc2x0 C., more preferably at a temperature of between 270 and 350xc2x0 C. Most preferably, the temperature is higher than 280xc2x0 C. because higher selectivities to xcex5-caprolactam and thus a higher overall yield to xcex5-caprolactam is obtained. The pressure of the gas phase process is preferably between 0.5 and 2 MPa, more preferably below 1.5 MPa.
The starting mixture comprising 6-aminocaproic acid, 6-aminocaproate ester, 6-aminocaproamide, 6-aminocapronitrile, oligomers of these compounds and/or polymers of these compounds can be obtained by various processes. For example in U.S. Pat. No. 4,730,040 a process is described in which an aqueous mixture is obtained containing 6-aminocaproic acid and some xcex5-caprolactam starting from 5-formylvalerate ester. Further in EP-A-729943 a process is described in which an aqueous mixture is obtained containing 6-aminocaproic acid, 6-aminocaproamide and xcex5-caprolactam also starting from a 5-formylvalerate ester. U.S. Pat. No. 5,068,398 describes a process in which an aqueous mixture is obtained containing 6-aminocaproate ester and some xcex5-caprolactam starting from a 5-formylvalerate ester. WO-A-9835938 describes a process in which an aqueous mixture is obtained containing xcex5-caprolactam and 6-aminocaproic acid and/or 6-aminocaproamide starting from 5-formylvaleric acid or from an alkyl 5-formylvalerate in water.
The starting compounds are preferably 6-aminocaproic acid and/or 6-aminocaproamide because high yields to xcex5-caprolactam are possible when starting from these compounds. Next to the 6-aminocaproic acid and/or 6-aminocaproamide some xcex5-caprolactam and oligomers of xcex5-caprolactam, 6-aminocaproic acid and/or 6-aminocaproamide can be present. A typical mixture which can be used as starting mixture for the present invention comprises between 5 and 50 wt. % 6-aminocaproic acid, 10 and 50 wt. % 6-aminocaproamide, 0 and 40 wt. % xcex5-caprolactam and between 0 and 35 wt. % of the earlier mentioned oligomers in which the total of these fractions is 100 wt. %.
Starting mixtures can also be obtained starting from 6-aminocapronitrile as for example described in WO-A-9837063. Processes starting from 6-aminocapronitrile can yield mixtures comprising 6-aminocaproic acid. Such mixtures can be advantageously used in the process according to the invention. Firstly, the 6-aminocapronitrile is contacted with water under hydrolysis conditions. Secondly, water and ammonia, which is formed in the hydrolysis step, are preferably separated. In a third step the resulting mixture is treated with the process according to the invention.
The process according to the invention can also be applied on aqueous starting mixtures containing 6-aminocapronitrile. Processes to prepare xcex5-caprolactam starting from 6-aminocapronitrile are for example described in U.S. Pat. No. 5,495,016 and EP-A-659741.
Other examples of starting mixtures which can be used in the process according to the invention are polycaprolactam processing waste, polycaprolactam carpet waste and/or polycaprolactam extraction wash water.
When the process of the invention is performed in the gaseous phase, the starting compound or mixture of starting compounds are preferably contacted with the superheated steam as a liquid, for example as a melt. Polycaprolactam waste is preferably fed to the reactor as a melt. This feeding may be achieved by using an extruder, gear pump, or other means known in the art.
The process according to the present invention can be applied with particular advantage using an aqueous mixture obtained in a previous process step, which mixture already contains 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers and/or polymers of these compounds. The aqueous mixture contains between 1 and 50 wt. % water, preferably between 1 and 20 wt. % water and between 50 and 99 wt. % of starting compound(s), preferably between 80 and 99 wt. % of starting compounds. An aqueous mixture containing 6-aminocaproic acid, 6-aminocaproamide, oligomers thereof and xcex5-caprolactam can with particular advantage be obtained through reductive amination of 5-formylvaleric acid or its ester as for example described in EP-A-729934 or WO-A-9835938.
The 6-aminocaproic acid, 6-aminocaproamide and 6-aminocaproic ester compound respectively are of the following formula: 
wherein R is OH, NH2 and OR1 respectively. R1 is as defined above.
The 6-aminocapronitrile compound is of the following formula:
H2Nxe2x80x94(CH2)5xe2x80x94Cxe2x89xa1N
With the process according to the invention a composition is obtained comprising (a) xcex5-caprolactam and (b) N-(5-carboxypentyl)-xcex5-caprolactam, wherein the amount of component (b) in the composition is higher than 0.1 wt. % and lower than 50 wt % (based on the total composition).
The process according to the invention is preferably performed as a continuous process. The xcex5-caprolactam can be separated from the reaction mixture obtained in the process of the invention using any separation technique known to a person skilled in the art. Examples of suitable separation techniques are crystallisation, (vacuum) distillation and/or extraction.
In a preferred embodiment, the purification of xcex5-caprolactam comprises the following steps:
1) feeding the xcex5-caprolactam product stream obtained in the process according to the invention to a partial condensation unit in which the product stream is split into a top stream comprising steam and a liquid bottom stream comprising xcex5-caprolactam, water, lights and heavies;
2) feeding the liquid bottom stream to a distillation column of which the top stream is mainly water and the bottom stream comprises xcex5-caprolactam, lights and heavies;
3) feeding the latter bottom stream to a vacuum distillation column of which the top stream is mainly lights and the bottom stream comprises xcex5-caprolactam and heavies;
4) feeding the latter bottom stream to a vacuum distillation column of which the top stream is the xcex5-caprolactam stream and the bottom stream is the heavies stream.
Compounds having a higher boiling point than xcex5-caprolactam are designated as heavies in this specification. Examples are 6-aminocaproic acid, 6-aminocaproamide, oligomers of these compounds, and N-(5-carboxypentyl)-xcex5-caprolactam. Compounds having a lower boiling point than xcex5-caprolactam are designated as lights in this specification. Examples are N-methyl-xcex5-caprolactam, hexanoic acid, 5-hexenoic acid, valeric acid and valeramide.
The xcex5-caprolactam streams resulting from the process according to the invention can be purified according to conventional techniques. Advantageously the caprolactam is purified by a crystallisation process as for example described in WO-A-9948867.
The condensation of the product stream (step 1) is preferably performed at a temperature of 80-200xc2x0 C., more preferably at a temperature of 100-170xc2x0 C.
The distillation in step 2) is for example performed at a temperature of 60-160xc2x0 C., preferably at a temperature of 80-140xc2x0 C.
The distillation in step 3) and 4) is preferably performed at a pressure lower than 10 kPa, more preferably at a pressure lower than 5 kPa. The temperature of the distillation in step 3) and 4) is preferably between 110-170xc2x0 C., more preferably between 120-150xc2x0 C.
It has been found that N-(5-carboxypentyl)-xcex5-caprolactam has a beneficial effect on the rate of reaction for preparing xcex5-caprolactam from 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers or polymers of these compounds or mixtures comprising at least two of these compounds. The invention therefore also relates to the use of N-(5-carboxypentyl)-xcex5-caprolactam as reaction rate enhancing compound in a process for preparing xcex5-caprolactam from 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, oligomers or polymers of these compounds or mixtures comprising at least two of these compounds.
The invention also relates to a process for the preparation of polyamide-6 starting from water and 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile, and/or xcex5-caprolactam which process in performed in the presence of N-(5-carboxypentyl)-xcex5-caprolacatam and/or derivative thereof in an amount of less than 50 wt % and more than 0.1 wt %. A process for the hydrolytic polymerisation of xcex5-caprolactam into polyamide-6 is for example described in Kunststoff Handbuch 3/4, Polyamide, Becker/Braun, Hanser Verlag, 1998, pages 41-47. A process for preparing polyamide-6 starting from 6-aminocapronitrile, and water is for example described in WO-A-9808889. It has been found that the presence of N-(5-carboxypentyl)-xcex5-caprolactam in such an amount has a beneficial effect on the rate of the hydrolytic polymerisation of 6-aminocaproic acid, 6-aminocaproamide, 6-aminocaproic ester, 6-aminocapronitrile and/or xcex5-caprolactam.
The invention will be elucidated by the following examples, however these are not intended to limit the scope of the invention in any way.